This invention relates to directional drilling systems. More specifically this invention relates to an improved method and apparatus for protecting a wireline in a directional drilling system.
In the history of drilling within the earth in search of natural resources, such as oil and/or gas, situations have arisen where it has been either necessary or desirable to drill at an angle with respect to an imaginary vertical reference extending downwardly from the rotary table of a rig.
In the above connection in drilling adjacent a salt dome it is often desirable to drill along a lateral side of the dome and then deviate beneath the dome into a production zone. In other instances a well may collapse or become plugged and it becomes necessary to drill around or sidetrack the disruption. Further, rotary drilling equipment tends to drift or curve to the right due to the clockwise rotation of the drill string. This drift often requires straightening in order to penetrate a target zone. Still further production zones can be located in relatively inaccessible locations such as beneath rivers, swamps, cities, mountains, etc. Along near shore regions it is sometimes desirable to rig up on shore and deviate the drill string outwardly to an offshore location. At locations remote from a shore line towers or platforms may be erected upon a water bed where limitations on platform mobility and collection and production considerations dictate drilling multiple wells from a single station. Still further semi-submersible and drillship activity in relatively deep waters often requires directional drilling to explore laterally from a single submerged wellhead. For these and other reasons the concept of directional drilling has precipitated considerable interest and activity over the years.
The practice of controlled directional drilling of oil wells traces back at least to 1933 and the California Huntington Beach field. The principal tools initially developed for directional drilling were the whipstock, the knuckle joint and the spudding bit. Of these early tools the whipstock has probably been most widely utilized and is still a standard in the industry.
The whipstock is in principle a long, inverted wedge which is concave on one side and when run into the bottom of a drill hole can be used to deflect and guide a rotary drill string off into a deviated course as a new hole is drilled. Further in this connection the whipstock or wedge shaped unit may be releasably secured to a drill pipe and drill bit by a shear pin and then run into a well hole. On bottom the pin shears and the wedge is used to deflect the path of the drill bit.
Although whipstocks have received a considerable amount of commercial utilization the art of controlled directional drilling was significantly enhanced by the development of a directional drilling string which includes a bent angle sub, a downhole motor and bit and a wireline steering tool. (As used throughout this application the term "sub" refers to a drill pipe substitute which is inserted into a drill string for some special purpose while concomitantly permitting the drill string to function in its conventional drilling capacity.) Wireline directional drill strings have enabled a directional driller to continuously monitor inclination, azimuth and toolface orientation and take corrective actions without interrupting a drilling operation.
In addition to the above identified elements or operative segments, most drill strings, and directional drill strings in particular, employ a jar within the string. A jar is used in drill strings to minimize the possibility of the string becoming irretrievably stuck in a well hole. Numerous situations, individually and/or in combination, can cause a drill string to stick such as improper drilling fluid viscosity, formation tackiness, well hole deviations, etc. The tendency for sticking is particularly relevant in wireline directional drilling operations where the drill string, per se, is not rotated on a deviation run. Additionally the drill string exhibits a considerable force normal to the walls of a sloping well bore. Further drilling activity is often interrupted in deviation runs thus the drill string occasionally lies idle in the well hole. In light of the above a jar is considered by many directional drillers to be critically necessary in a wireline directional drill string.
While there are numerous designs for drilling jars most units include a hammer element and an anvil interconnected by some trip or releasable mechanism. In operation a generally predetermined lift or weight is placed on the drill string. This force causes the trip to release and the hammer strikes the anvil with an accelerating blow sufficiently violent to shake or jar the entire drill string. The operative stroke of a jar is provided by a telescoping spline element and typically the stroke of a jar will be twenty two or so inches in length.
In a wireline directional drilling string the wireline extends from the steering tool upwardly through the interior of the jar and onto the surface. The wireline is composed of an electrical conductor surrounded by an insulator and a protective wire braided sheath for strength. At the surface, data from the steering tool is continuously processed as the new hole advances as indicated above.
Although wireline directional systems are highly advantageous a significant problem exists for directional drillers using wireline systems of the type previously described. In this regard although the jar is set to "go off" at a predetermined amount of push or pull the exact timing is not accurately predictable. Moreover the acceleration of the hammer into the anvil, which is necessary to produce an effective shake or jar to the string, has a tendency to kink or even in some instances snap the wireline. When kinking occurs, communication with the steering tool tends to become intermittent to non-existent. In such event directional drilling must be shut down and the steering tool must be pulled and rewired before the directional run can proceed. Of course any down time on a rig is quite expensive and can quickly mount up to several thousand dollars.
The foregoing problem of kinking and re-running wireline has become so critical that in some instances directional drillers have gambled on the drill string not becoming struck and running the directional string without utilizing a jar. Of course if the string does become struck significant downtime is inevitable as a sidetracking run is made.